Study Finds Brain Mass and Body Mass Follow a Curved Relationship

The notion that an animal’s brain mass is linked to its body mass seems intuitive: bigger animals generally have larger brains than their smaller relatives.

Yet researchers have struggled to establish a conclusive model showing precisely how these two measurements are connected.

A study published in Nature Ecology and Evolution this month argues that the difficulty stems from a basic flaw in a long-standing assumption about the mathematical link between the two masses.

Brain mass and body mass in mammals

The research puts forward a different model that not only matches the available data more closely, but could also help explain several enduring questions about brain evolution.

For decades, the usual view has been that body mass and brain mass are related by a fairly straightforward power law. This suggested relationship has remained strongly contested, however, because it appears not to hold across every species group.

So far, scientists have not identified a version of the equation that applies universally. Differences between taxonomic levels have proved particularly troublesome, to the extent that they have acquired their own label: the taxon-level problem.

Although researchers have extensively debated both the value and origin of a crucial part of the relationship, known as the allometric component, the new paper considers a more basic issue. It asks whether the assumption that brain mass and body mass follow any form of logarithmic linear relationship is correct in the first place.

To investigate, the authors analysed a substantial dataset containing 1,504 mammalian brain/body mass measurements, assessing which model provided the closest fit.

A curving relationship rather than a straight line

Their results showed that the relationship is not log-linear but log-curvilinear: on a logarithmic plot, the line needs to bend to reflect the data.

The researchers tried several equations to produce this curve. The best match was a second-order polynomial, a type of quadratic equation commonly encountered at secondary school.

Its central characteristic is that “as mammals increase in mass, the rate at which brain mass increases with body mass decreases”. The model therefore predicts that exceptionally large animals have smaller brains than a linear model would anticipate - precisely the pattern visible in the dataset.

A single curve for fitting the brain/body relationship also enables comparisons between groups that produced notably different allometric coefficients under the earlier model.

Evolutionary patterns in larger brains

The study illustrates a benefit of the revised approach: “[it] makes it possible to study evolutionary trends in trait (or relative trait) evolution through time.” It consequently considers how quickly brain mass rises - in other words, the pace at which larger brains evolve - and identifies substantial variation among species.

As might be expected, primates developed large brains particularly rapidly. Rodents and carnivores did so as well. Just three animal groups displayed an increasing brain-to-body-mass relationship over time.

Although the research focuses on mammals, the authors also assessed a dataset of bird body/brain mass pairs. The curvilinear relationship fitted those data well too. Perhaps the most compelling unresolved question, though, is why brain mass and body mass are correlated in this way.

This remains an open question, and the answer could reveal much about how and why animals have evolved as they have.

As the study concludes, “seeking both the theoretical and empirical underpinning for curvilinear relationships across species will probably lead to major contributions across biology.”

This research was published in Nature Ecology and Evolution.